\(v_E(x)\)  is called the generalized exterior normal to \(E\)  at \(x\in \partial ^{\ast }E\) . By [1]}, if \(E\in \mathcal {C}^n\) , \(\partial ^{\ast } E\)  is \(n-1\) -rectifiable, i.e., it can be covered, except for an \(\mathcal {H}^{n-1}\) -negligible subset, by countably many \((n-1)\) -dimensional surfaces of class \(C^1\) .
